pruning a prunus cerasifera

I have a 4 meter high 15 year old purple leaf plum prunus cerasifera in my garden and I have never dared to prune it except for removing an occasional damaged branch. the last couple of years it is hardly producing any plums anymore (it used to carry tons of them) and the tree is carying loads of very thin fragile branches. Research on the web reveals contradictory information; one webpage even saying that the maximum age of such a tree is 20 years and that it starts declining after 15 years...

the last couple of years there have not been that many flowers, and the tree flourished far to soon ie. early march, so not a lot of bees around and then in march and april the stormy winds did the rest of the job ( I live at the sea-side) ...the bottomline is that there were not a lot of flowers: could this be caused by the lack of pruning ?

Cherry plums flower on newish timber so the reason may be lack of growth. It sounds as if the tree is full of weak twiggy stuff - another sign of poor growth. Pruning generally reduces flowering and fruiting but removal of weak twiggy growth within the canopy of the tree and boosting growth by removal of competing vegetation and mulching with garden compost might produce sufficient growth for flowering to resume without the tree getting to rampant.

This year the weather when they were flowering was rather wet and cold so fruiting won't be very good - for the last two years conditions were quite good at flowering and fruit production was above average.

Purple leaves are very costly for plants to manufacture so any adversity reduces cropping in purple leaved plants more than in green ones.

the way you describe it is exactly what the tree looks like: loads and loads of very small delicate branches - 'twiggy stuff' is indeed the correct expression. and there are some other smal scrubs at the base of the tree, so I should get rid of them as well. If I understand you well removing the mass of little twigs could help; but how do I do this ; - when is the best time / during what month, - do I take it out spread over a couple of years or can I do it all in just one 're-juvenating' operation,- do I cut the twiggy stuff back to the main branches or do I keep 12 inches / 30 cm of the main twigs ?If this implies that the tree will not produce any fruit next year then so be it; I rather have it surviving in a healthy way then seeing it perish slowly ( which my son would hate to see since he grew up with that tree, but that is another story...)